Who were The most powerful Indian tribe in the Missouri river valley in nineteenth century?

The most powerful Indian tribe in the Missouri River Valley during the nineteenth century was the Sioux, particularly the Lakota subgroup. They dominated the region through their formidable warrior culture and strategic alliances, often resisting encroachment by European settlers and the U.S. government. The Sioux played a crucial role in the conflicts of the era, including the Indian Wars, and their influence extended across a vast territory in the Great Plains. Their resistance was marked by notable events such as the Battle of Little Bighorn in 1876.